US11913683B2 - Solid-state laser refrigeration of composite optomechanical resonators - Google Patents

Abstract

An apparatus and method of indirectly cooling an optomechanical resonator, comprising impinging a laser on an optomechanical resonator attached to a substrate, wherein the optomechanical resonator comprises a cantilever, a cooling end of the cantilever, having a cooling end comprising a laser-induced cooling element, an attachment end of the cantilever, attached to a substrate, and wherein the laser has a peak wavelength in the near-infrared band.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Patent Application No. 62/962,794, filed Jan. 17, 2020, the disclosure of which is hereby incorporated by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with government support under Grant No. FA9550-16-1-0362, awarded by the Air Force Office of Scientific Research. The government has certain rights in the invention.

BACKGROUND

Photothermal heating represents a major constraint that limits the performance of many nanoscale optoelectronic and optomechanical devices including nanolasers, quantum optomechanical resonators, and integrated photonic circuits. Although radiation-pressure damping has been reported to cool an individual vibrational mode of an optomechanical resonator to its quantum ground state, to date the internal material temperature within an optomechanical resonator has not been reported to cool via laser excitation.

Additionally, optically refrigerating the lattice of a dielectric resonator has the potential to impact several fields including scanning probe microscopy, the sensing of weak forces, the measurement of atomic masses, and the development of radiation-balanced solid-state lasers. In addition, optically refrigerated resonators may be used in the future as a promising starting point to perform motional cooling for exploration of quantum effects at mesoscopic length scales, temperature control within integrated photonic devices, and solid-state laser refrigeration of quantum materials.

Further, photothermal heating is a perennial challenge in the development of advanced optical devices at nanometer length scales given that a material's optical index of refraction, bandgap, and Young's modulus all vary with temperature. For instance, reducing the mechanical motion of an optomechanical resonator to its quantum ground state requires that the temperature (T) must be much less than hν/kb, where ν is the mode frequency, h and kb are Planck and Boltzmann constants, respectively. Critically, incident laser irradiances must be kept low enough to avoid photothermal heating of the resonator above cryogenic temperatures.

Among other applications in scanning probe microscopy and exploration of quantum effects at mesoscopic length scales, optical refrigeration of a mechanical resonator could have significant implications for weak force and precision mass sensing applications, in the development of composite materials for radiation balanced lasers, and local temperature control in integrated photonic devices. In the future, solid state laser refrigeration may also assist in the cooling of optomechanical devices by enabling the use of higher laser irradiances in the absence of detrimental laser heating.

Because of this, optomechanical resonators configured to cool, and methods for cooling optomechanical resonators are needed.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

In one aspect, an optomechanical resonator is provided that is configured to indirectly cool via laser irradiance, the optomechanical resonator comprising:

• • a cantilever;
  • a cooling end of the cantilever, having a cooling end comprising a laser-induced cooling element; and
  • an attachment end of the cantilever, attached to a substrate.

In another aspect a method of indirectly cooling an optomechanical resonator is provided. In one embodiment the method includes impinging a laser on an optomechanical resonator attached to a substrate, wherein the optomechanical resonator comprises:

• • a cantilever;
  • a cooling end of the cantilever, having a cooling end comprising a laser-induced cooling element; and
  • an attachment end of the cantilever, attached to a substrate; and
  • wherein the laser has a peak wavelength in the near-infrared band.

In some embodiments, an optomechanical resonator is described. In some embodiments, the semiconductor optomechanical resonator is suspended in vacuum from a silicon wafer to reduce the potential for photothermal heating of the adjacent silicon substrate. In some embodiments, Van der Waals bonding is used to attach a low-cost, hydrothermal ceramic Yb:YLF microcrystal to the end of the resonator cavity.

In some embodiments, rare-earth (Yb³⁺) point-defects within the YLF emit anti-Stokes photoluminescence which cools both the YLF microcrystal, and the underlying semiconductor optomechanical resonator. The YLF may serve both as a local thermometer and also as a heat sink which extracts thermal energy from the cantilever, increasing its Young's modulus, and thereby blue-shifting the cantilever's optomechanical eigenfrequency. In some embodiments, the transmitted laser causes minimal heating of the cantilever supporting the YLF crystal due to its small thickness (˜150 nm) and extremely low absorption coefficient of CdS at 1020 nm.

In some embodiments, the direct laser refrigeration of a semiconductor optomechanical resonator is >20K below room temperature based on the emission of upconverted, anti-Stokes photoluminescence of trivalent ytterbium ions doped within a yttrium-lithium-fluoride (YLF) host crystal. In some embodiments, a photothermal cooling of nanoscale optoelectronic devices through the emission of blue-shifted (anti-Stokes) photoluminescence is disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings wherein:

FIG. 1A is a schematic of an example optomechanical resonator in accordance with the present technology;

FIG. 1B is a schematic of an eigenfrequency and upconverted fluorescence measurement setup in accordance with the present technology;

FIG. 1C is a bright field optical image of an example optomechanical resonator in accordance with the present technology;

FIG. 1D is a graph of a peak in the thermomechanical noise spectrum originating from the fundamental eigenfrequency of an example optomechanical resonator in accordance with the present technology;

FIG. 1E is a graph of an example stitched, up-converted fluorescence spectrum obtained at room temperature in accordance with the present technology;

FIG. 2A is a graph of normalized power spectra for a representative laser refrigeration measurement at an example laser irradiance in accordance with the present technology;

FIG. 2B is a graph of the frequency shift with laser power of an example optomechanical resonator in accordance with the present technology;

FIG. 2C is a graph of temperature calibration of an example optomechanical resonator in accordance with the present technology;

FIG. 3A is the geometry of an exemplary optomechanical resonator system used for analytical and finite element heat transfer modeling;

FIG. 3B is a graph of the steady state temperature along the length of an example cantilever in accordance with the present technology.

DETAILED DESCRIPTION

The present technology demonstrates an approach to refrigerate the temperature of an optomechanical resonator >20K below room temperature using solid state laser refrigeration.

In one aspect, an optomechanical resonator is provided that is configured to indirectly cool via laser irradiance, the optomechanical resonator comprising:

• • a cantilever;
  • a cooling end of the cantilever, having a cooling end comprising a laser-induced cooling element; and
  • an attachment end of the cantilever, attached to a substrate.

In another aspect a method of indirectly cooling an optomechanical resonator is provided. In one embodiment the method includes impinging a laser on an optomechanical resonator attached to a substrate,

• • wherein the optomechanical resonator comprises:
  • a cantilever;
  • a cooling end of the cantilever, having a cooling end comprising a laser-induced cooling element; and
  • an attachment end of the cantilever, attached to a substrate; and
  • wherein the laser has a peak wavelength in the near-infrared band.

FIG. 1A is a schematic of an example optomechanical resonator 100 in accordance with the present technology. The optomechanical resonator 100 may be a portion of a device including, but not limited to, a scanning probe microscope, a photonic waveguide device, a photonic detector device, a nonlinear photonic device, an electronic device, a force sensor, or a mass sensor. The optomechanical resonator 100 includes a cantilever 105, a cooling end 110, and an attachment end 115. In some embodiments, the optomechanical resonator 100 may be configured to indirectly cool, as described below. In some embodiments, the optomechanical resonator 100 indirectly cools using laser irradiance.

The optomechanical resonator 100 includes a cantilever 105. In some embodiments, the cantilever 105 is formed from a material having a low absorption coefficient at a peak wavelength. In some embodiments, the peak wavelength is the peak wavelength of a laser, such as cooling element 110, described in further detail below. In some embodiments, the peak wavelength is in the range of 1020 nm to 1064 nm. In some embodiments, the cantilever 105 is formed from CdS, Si₃N₄, and/or SiO₂. In some embodiments, the cantilever 105 has a flat, smooth surface. The cantilever 105 is illustrated as rectangular in FIG. 1A, but in other embodiments, the cantilever 105 may take any number of shapes.

In some embodiments, the optomechanical resonator 100 includes a cooling end 110. The cooling end 110 may be attached to the cantilever 105 by Van der Waals bonding. In some embodiments, the cooling end 110 is a laser-induced cooling element. Examples of such a laser-induced cooling element include, but are not limited to, Yb:YLiF4, Yb:NaYF4, Yb:LuLiF4, Yb:KLuF4, and Yb:KYF4 crystals. In some embodiments, the laser-induced cooling element has a flat surface. In some embodiments, the cooling end 110 is cuboid, but in other embodiments, the cooling end 110 may take any number of shapes.

In some embodiments, the optomechanical resonator 100 includes an attachment end 115. In some embodiments, the attachment end 115 is attached to a substrate (not pictured in FIG. 1A). In some embodiments, the substrate is a silicon substrate. In some embodiments, the attachment end 115 is formed from the substrate.

In operation, the cooling end 110 impinges the optomechanical resonator 100 with laser irradiance. In some embodiments, the cooling results from the emission of upconverted, anti-Stokes photoluminescence in the laser-induced cooling element. In some embodiments, the laser has a peak wavelength in the near-infrared band. In some embodiments, the laser irradiance has a peak wavelength in the range of 1020 nm to 1064 nm. In some embodiments, the cooling end 200 cools the optomechanical resonator 1000 to a temperature greater than 20K.

In certain embodiments the cooling end 110 has a thickness in the range of 200 nm to 10 micrometers. In a further embodiment, the cooling end has a thickness in the range of 2 micrometers to 6 micrometers.

In certain embodiments, the cantilever has nanoscale dimensions that enable thermomechanical motion. This is a unique nanoscale effect that enables the cantilever to be “driven” by heat (rather than an external piezo stage) as a consequence of the fluctuation-dissipation theorem of statistical thermodynamics. It is analogous to Brownian motion, but for a small solid object that does not have to be in a fluid. With thermomechanical motion, an additional piezo stage is not needed to drive or sense the cantilever's eigenfrequency.

In relation to the nanoscale effects described herein, in certain embodiments the cantilever 105 has a thickness that is 200 nm or less. In a further embodiment, the cantilever has a thickness of 150 nm or less. In yet a further embodiment, the cantilever has a thickness of 80 nm to 150 nm. In yet a further embodiment, the cantilever has a thickness of 80 nm to 200 nm.

If the cantilever 105 is thicker than 200 nm it may not have the thermomechanical motion, but certain embodiments described herein do not require thermomechanical motion, such as an AFM cantilever driven by a piezo instead of thermomechanical motion. In such embodiments, the cantilever 105 has a thickness of between 200 nm and 5 micrometers. In a further embodiment, the cantilever has a thickness between 1 micrometer and 3 micrometers.

FIG. 1B is a schematic of an eigenfrequency and upconverted fluorescence measurement setup in accordance with the present technology. The example measurement setup includes focusing lenses (FL), a mirror (M), a short pass filter (SPF), a diode laser (DL), a beam splitter (BS), a variable neutral density filer (VND) and an avalanche photodiode (APD). In some embodiments, the short pass filter (SPF) is a 1000 nm SPF. An optomechanical resonator (such as optomechanical resonator 100) is illustrated (Sample). The optomechanical resonator may include a cantilever (such as cantilever 105) a cooling element (such as cooling end 110), and an attachment end (such as attachment end 115). In some embodiments, the cooling element is a Yb:YLF crystal.

In one embodiment, a micron-scale grain of 10% Yb³⁺-doped YLiF₄ (Yb:YLF) located at the end of a semiconductor optomechanical resonator (CdS) to cool the resonator >20K below room temperature was used, following excitation with a cw-laser source with wavelength λ₀=1020 nm.

In operation, the measurement setup in FIG. 1B measures the voltage vs. time signal of the cantilever. The eigenfrequencies of the cantilever may then be calculated by Fourier transforming the voltage vs. time signal to obtain its thermomechanical noise spectrum. In operation, a laser may be focused onto the cooling element at the end of the cantilever. In some embodiments, the laser is a 1020 nm laser. In some embodiments, the 1020 nm laser is forward scattered. In some embodiments, the time-dependent intensity of the forward-scattered 1020 nm laser is measured by focusing it onto the APD.

In one embodiment, the backscattered photoluminescence was collected from the rear end of the objective, transmitted through a beam splitter, and filtered using a 1000 nm short-pass filter and focused into the spectrometer slit. The photoluminescence (PL) spectrum was recorded at different grating positions, with appropriate collection times to avoid saturating the detector, and were stitched together. Ten spectra were collected using 39 kW/cm² of laser irradiance for 0.1 s and averaged (as shown in FIG. 1D). A longer acquisition time (50×) was used to collect the weaker luminescence signal from the other rare earth (RE) impurities that were not explicitly added during synthesis.

FIG. 1C is a bright field optical image of an example optomechanical resonator in accordance with the present technology. The illustrated optomechanical resonator includes a CdSNR cantilever supported using a silicon substrate with a cooling end in the form of a Yb³⁺:YLF crystal placed at the free end of the cantilever. In other embodiments, the cantilever, substrate, and cooling end may be comprised of other materials. In one embodiment, a CdSNR cantilever was placed at the end of a clean silicon substrate, and a hydrothermally grown 10% Yb:YLF crystal was placed at the free end of the CdSNR cantilever. The cantilever may be formed of CdS because of its wide band gap and low-cost, though any material with low near-infrared (NIR) absorption can be used. In one embodiment, the silicon substrate was loaded inside a cryostat chamber such that the free end of the cantilever was suspended over the axial hole in the cryostat, and the system was pumped to ˜10⁻⁴ torr.

FIG. 1D is a graph of a peak in the thermomechanical noise spectrum originating from the fundamental eigenfrequency of an example optomechanical resonator in accordance with the present technology. On the horizontal axis is the Frequency. On the vertical axis is the Power. A representative power spectrum measured on the sample at 300 K using a laser irradiance of 39 kW/cm² is shown, generated from the optomechanical resonator described in FIG. 1C.

Illustrated is a peak in the thermomechanical noise spectrum originating from the fundamental eigenfrequency of the CdSNR with Yb³⁺:YLF sample obtained at the 0.038 MW/cm² at 300 K. The sharp peak, fitted using a standard Lorentzian with a peak position at 3648.9 Hz, was attributed to the first natural resonant frequency mode (“diving board mode”) of the fluoride crystal on the nanoribbon (FCNR) cantilever system.

FIG. 1E is a graph of an example stitched, up-converted fluorescence spectrum obtained at room temperature in accordance with the present technology. On the horizontal axis is the Wavelength. On the vertical axis is the Intensity. The example stitched, up-converted fluorescence spectrum was obtained at room temperature using a 1020 nm excitation source (0.039 MW/cm²) focused on the suspended Yb³⁺:YLF crystal. A 1000 nm short pass filter was used to cut off the laser line.

The emission of upconverted, anti-Stokes photoluminescence of trivalent ytterbium ions doped within an yttrium-lithium-fluoride (YLF) host crystal is illustrated. The intense Yb³⁺ transitions in the range of 800 to 1000 nm, with major peaks at 960 (E₆−E₁), 972 (E₅−E₁) and 993 (E₅−E₃) nm were observed. The up-converted green and red emission peaks at 520, 550 and 650 nm are attributed to the transitions from ²H_11/2, ⁴S_3/2 and ⁴F_9/2, respectively, of trivalent erbium ions (Er³⁺). Other minor transitions are labeled.

FIG. 2A is a graph of normalized power spectra for a representative laser refrigeration measurement at an example laser irradiance in accordance with the present technology. On the horizontal axis is the Frequency Shift. On the vertical axis is the Normalized Power. The arrow along the top of the graph shows increasing irradiance.

The example graph shows normalized power spectra for a representative laser refrigeration measurement at each laser irradiance with an ambient reference temperature of 295 K (f₀=3632.2 Hz). Power spectra normalized using the maximum value at different laser irradiances obtained from the sample of one embodiment of the present technology are plotted. When fit to a standard Lorentzian, the peak values show a blue-shift in the eigenfrequency of the FCNR system as the laser power is increased.

FIG. 2B is a graph of the frequency shift with laser power of an example optomechanical resonator in accordance with the present technology. On the horizontal axis is Irradiance. On the vertical axis is the Frequency Shift (f−f₀) with laser power at 295K. One data series represents a plain CdSNR cantilever (without a cooling end). The other data series represents a CdSNR cantilever with a cooling end in the form of a Yb:YLF crystal. The fitted peak values of the power spectra in FIG. 2A are shown in FIG. 2B. Each data point was averaged from Lorentzian fits of 6 power spectra and error bars represent one standard deviation. Note that for small standard deviations, the error bars overlap with the data point; f₀ is 3632.2 Hz and 17384.4 Hz, respectively.

As the laser irradiance increases, the Yb:YLF source reaches lower temperatures, thereby extracting more heat from the CdSNR cantilever and causing a blue-shift in the frequency due to an increased Young's modulus of the CdS at lower temperatures.

In one embodiment, using 0.5 MW/cm² of 980 nm laser resulted in the irreversible photothermal melting of the cantilever device. When the Yb:YLF crystal was removed from the CdSNR cantilever, the fundamental frequency measured at 39 kW/cm² increased to a higher value of 17384.3 Hz due to the removal of mass from the system (˜1.3×10⁻⁹ g).

In one embodiment, as a control experiment, the eigenfrequency of the CdSNR cantilever itself was measured after the removal of the Yb:YLF crystal. The eigenfrequency of the cantilever without the crystal was then measured as a function of the laser power and is shown in FIG. 2B. The eigenfrequency red-shifts as the laser irradiance is increased, suggesting greater heating of the cantilever at higher irradiances due to the decreasing Young's modulus at higher temperatures.

In one embodiment, the temperature of the FCNR device was calibrated by increasing the temperature of the cryostat from 160 to 300 K, which showed a linear red-shift in the eigenfrequency of the cantilever. The slope of −0.389 Hz/K obtained using this calibration was used to measure the temperature change of the cantilever system during laser refrigeration experiments. The maximum blue-shift in the eigenfrequency as a function of laser irradiance of the was +20.6 Hz at an irradiance of 965 kW/cm², compared to the lowest irradiance of 39 kW/cm². Based on the isothermal temperature calibration, ignoring temperature gradients and other optomechanical effects on the cantilever due to increased irradiance, this blue-shift of +20.6 Hz corresponds to a temperature change of 53 K below room temperature (assuming a negligible change in temperature at a laser irradiance of 39 kW/cm²).

FIG. 2C is a graph of temperature calibration of an example optomechanical resonator in accordance with the present technology. On the horizontal axis is Temperature in K. On the vertical axis is the Frequency Shift (f−f₀). The example temperature calibration graph was generated by measuring the frequency shift (f₀=3653.6 Hz) as a function of the cryostat temperature.

However, to obtain the absolute change in temperature it is important to view the system through a modified Euler-Bernoulli beam theory and include the effects of the laser trapping forces on the Yb:YLF crystal, which acts as a spring at the end of the cantilever. With increased irradiances, due to the increased force constant of the spring, the eigenfrequency of the cantilever increases. Analytically the eigenfrequency (f_i) in hertz, of a uniform rectangular beam is given by:

$\begin{array}{cc}{f}_i=\frac{1}{2\pi }\frac{{\Omega }_i^2}{{\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="US11913683-20240227-D00004.png"\; state="\{\{state\}\}">L</figure-callout>}}^2}\sqrt{\frac{E1}{\rho }}.& \left(1\right)\end{array}$

Here L is the length, ρ is the linear density, E is the Young's modulus, and I is the area moment inertia of the cross section of the beam. The i^th eigenvalue of the non-dimensional frequency coefficient Ω_i satisfies the following equation for a uniform rectangular cantilever with a mass Mo and spring of spring constant K attached at the free-end of the cantilever of mass mo.

$\begin{array}{cc}-\left(\frac{K}{{\Omega }_i^3}-\frac{{\Omega }_i{M}_0}{m_0}\right)\left[\cos \left({\Omega }_i\right)\sin h\left({\Omega }_i\right)-\sin \left({\Omega }_i\right)\cos h\left({\Omega }_i\right)\right]+\cos \left({\Omega }_i\right)\cos h\left({\Omega }_i\right)+1=0.& \left(2\right)\end{array}$

In one embodiment, to experimentally probe the effects of the laser trapping forces, the power-dependent eigenfrequency measurements were performed at a constant cryostat temperature of 77 K. At temperatures as low as 77 K, the cooling efficiency of the Yb:YLF crystal decreases due to diminishing resonant absorption and red-shifting of the mean fluorescence wavelength. Due to negligible cooling with increased irradiance, and with the equilibrium temperature being maintained by the crysostat, it is assumed that any blue-shift in the eigenfrequency of the system was solely due to the greater laser trapping force at higher irradiance. Therefore, the excessive blue-shift at room temperature (6+/2.2 Hz) can be attributed to the change in Young's modulus due to cooling of the CdSNR cantilever. According to this calibration, the cantilever's temperature is reduced 15.4+/−5.6 K below room temperature. Since cantilever eigenfrequencies are calibrated at isothermal conditions, the temperatures measured via cantilever eigenfrequencies during laser-refrigeration do not directly measure the coldest point within the cantilever, but rather a lower bound of the absolute minimum achievable temperature decrease. This is a consequence of temperature gradients within the cantilever that lead to gradients of the cantilever's Young's modulus. Based on finite element eigenfrequency modeling of the cantilever with a spatially varying Young's modulus, the coldest point in the cantilever can be calculated.

Example

Heat Transfer Analysis

Below, a steady state, heat-transfer model of the laser-cooled cantilever system to quantify how thermal gradients within CdSNR cantilevers affect eigenfrequency measurements during laser cooling experiments is presented.

FIG. 3B is a graph of the steady state temperature along the length of an example cantilever (illustrated in FIG. 3A) in accordance with the present technology. On the horizontal axis is the position (x) along the length of the cantilever, where position x=0 is at the attachment end of the cantilever, attached to the substrate. On the vertical axis is the Temperature (T) in Kelvins. As the position along the cantilever is moved farther from the substrate, the Temperature drops, eventually leveling out at −23.6K.

The steady state temperature along the length of the CdSNR was calculated using analytical one-dimensional solution obtained assuming all of the cooling power produced by the YLF crystal flows through the CdSNR cross section at L_c.

The YLF crystal is approximated as a cuboid with sides of H_c=6, L_c=7.5 and W_c=6 μm, such that the volume and aspect ratio was similar to the tetragonal bi-pyramidal YLF crystal used experimentally.

At steady state, the temperature distribution in the nanoribbon satisfies the energy equation given by:

$\begin{array}{cc}\frac{{\partial }^{2}T}{\partial x^2}+\frac{{\partial }^{2}T}{\partial {\mathrm{<figure-callout\; id="2"\; label="y"\; filenames="US11913683-20240227-D00004.png"\; state="\{\{state\}\}">y</figure-callout>}}^2}+\frac{{\partial }^{2}T}{\partial x^2}=0.& \left(3\right)\end{array}$

Heat transfer to the surroundings by conduction and convection is absent due to the vacuum surrounding the cantilever. Radiant (blackbody) energy transfer to or from the surroundings is negligible due to the relatively low temperatures of the cantilever and its small surface area. Therefore, the heat flow within the cantilever is one-dimensional and equation (3) reduces to:

$\begin{array}{cc}\frac{{\mathrm{<figure-callout\; id="2"\; label="d"\; filenames="US11913683-20240227-D00004.png"\; state="\{\{state\}\}">d</figure-callout>}}^2\mathrm{<figure-callout\; id="2"\; label="T\; dx"\; filenames="US11913683-20240227-D00004.png"\; state="\{\{state\}\}">T</figure-callout>}}{{\mathrm{dx}}^{}}& \left(4\right)\end{array}$

which has the general solution:
T(x)=C ₁ x+C ₂.  (5)

Assuming negligible interfacial resistance between the cantilever and the underlying silicon substrate, the temperature at the silicon/CdS interface (x=0) is the cryostat temperature T₀. Consequently, the boundary condition at the base of the nanoribbon is T(0)=T₀. If all of the heat generated in the YLF crystal is transferred to or from the CdSNR across the interface at x=L_c, the heat flux at the interface is given by:

$\begin{array}{cc}\kappa \frac{\mathrm{dT}}{\mathrm{dx}}\left(L_c\right)=\frac{{\stackrel{.}{Q}}_c}{\mathrm{HW}},& \left(6\right)\end{array}$

in which Q′_c is the rate of heat removal from the YLF crystal, and κ is the thermal conductivity of CdS.

Applying the boundary conditions, the temperature distribution in the CdSNR becomes:

$\begin{array}{cc}T\left(x\right)=\frac{{\stackrel{.}{Q}}_c}{\kappa \mathrm{HW}}x+{\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="US11913683-20240227-D00004.png,US11913683-20240227-D00005.png"\; state="\{\{state\}\}">T</figure-callout>}}_0.& \left(7\right)\end{array}$

It is assumed that the relatively large thermal conductivity of the YLF crystal (˜6 W/m·K) will lead to a nearly uniform temperature in the crystal given by:

$\begin{array}{cc}T\left(L_c\right)=\frac{{\stackrel{.}{Q}}_c}{\kappa \mathrm{HW}}{L}_c+{\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="US11913683-20240227-D00004.png,US11913683-20240227-D00005.png"\; state="\{\{state\}\}">T</figure-callout>}}_0.& \left(8\right)\end{array}$

The local rate of laser energy absorbed per unit volume Q′″=Q_abs/V is given by:

$\begin{array}{cc}{Q}^{\mathrm{\prime \prime \prime }}=\frac{4\pi n^{\prime }{n}^{\mathrm{\prime \prime }}}{{\mathrm{<figure-callout\; id="0"\; label="\lambda "\; filenames="US11913683-20240227-D00004.png,US11913683-20240227-D00005.png"\; state="\{\{state\}\}">\lambda </figure-callout>}}_0{\mathrm{<figure-callout\; id="0"\; label="Z"\; filenames="US11913683-20240227-D00004.png,US11913683-20240227-D00005.png"\; state="\{\{state\}\}">Z</figure-callout>}}_0}\left(E·E^{*}\right).& \left(9\right)\end{array}$

Here n=n′−in″ is the complex refractive index of the medium, λ₀ is the wavelength in vacuum, Z₀ is the free space impedance (Z₀=376.73 ohms), and E* is the complex conjugate of the local electric field vector within the YLF crystal. Upconverted, anti-Stokes luminescence follows laser absorption, cooling the crystal. The absorption of the incident laser by the underlying CdS cantilever is neglected due to its small thickness (154 nm) and low absorption coefficient at 1020 nm (6.7×10⁻¹³ cm⁻¹) relative to what has been reported for YLF (˜1 cm⁻¹).

Given that eigenfrequency measurements can only provide a lower bound of the cantilever's temperature, a more direct approach must be used to measure the temperature at the end of the cantilever. Differential luminescence thermometry (DLT) can be used to measure the temperature of the YLF at the end of the cantilever based on using a Boltzmann distribution to analyze emission from different crystal field (Stark) levels. DLT was used to measure a temperature drop of 23.6 K below room temperature (ΔT_max) at an irradiance (I₀) of 965 kW/cm² corresponding to an incident power P₀=40.1 mW and spot radius w₀=1.15 μm. Using the measured value of T(L_c)−T₀=23.6 K, H=150 nm, W=2.5 μm, L_c=53 μm, and κ=20 W/(m K), a cooling power of Q′_c=3.34×10⁻⁶ W is calculated. The resultant temperature gradient along the length of the device is shown in FIG. 3 . Based on the temperature gradient, by modeling a spatially varying Young's modulus, the coldest point in the cantilever from eigenfrequency measurements was calculated to be between 26 and 58 K below room temperature. This agrees well with the coldest temperature measured using DLT.

An absorption coefficient and cooling efficiency of 0.61 cm⁻¹ and 1.5%, respectively, have been reported previously for a bulk YLF crystal doped with 10% Yb-ions. Based on this absorption coefficient, a Yb:YLF crystal with a thickness of 6 μm would generate, to first order, a maximum cooling power of 0.22 μW when irradiated by a pump laser with a power of 40.1 mW. This power is an order of magnitude smaller than the experimental cooling power reported above. The discrepancy can be explained by two factors related to the symmetric morphology of the YLF microcrystals. First, the size of the YLF microcrystals is within the Mie-regime for light scattering and internal optical fields may be enhanced considerably due to morphology dependent cavity resonances.

A modified Euler-Bernoulli model was used to account for the laser trapping forces, and the measured temperatures were validated using heat transfer theory. A maximum drop in temperature of 23.6 K below room temperature was measured near the tip of the cantilever.

Claims (17)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of indirectly cooling an optomechanical resonator, comprising impinging a laser on an optomechanical resonator attached to a substrate,

wherein the optomechanical resonator comprises:

a cantilever;

a cooling end of the cantilever, comprising a laser-induced cooling element;

wherein the laser induced cooling element comprises a microcrystal cooling material within the Mie-regime for light scattering; and

an attachment end of the cantilever, attached to a substrate; and

wherein the laser has a peak wavelength in the near-infrared band.

2. The method of claim 1, wherein the cantilever is formed from a material having an absorption coefficient of 6.7×10⁻¹³ cm⁻¹ to 0.61 cm⁻¹ at the peak irradiance wavelength.

3. The method of claim 1, wherein the cantilever is formed from a material selected from the group consisting of CdS, Si₃N₄, and SiO₂.

4. The method of claim 1, wherein the cantilever has a flat, smooth surface.

5. The method of claim 1, wherein the laser-induced cooling element is attached to the cantilever by Van der Waals bonding.

6. The method of claim 1, wherein the laser-induced cooling element includes a material selected from the group consisting of Yb:YLiF₄, Yb:NaYF₄, Yb:LuLiF₄, Yb:KLuF₄, and Yb:KYF₄.

7. The method of claim 1, wherein the laser-induced cooling element has a flat surface.

8. The method of claim 1, wherein the optomechanical resonator is cooled greater than 20K.

9. The method of claim 1, wherein the laser has a peak wavelength in a range of about 1020 nm to about 1064 nm.

10. The method of claim 1, wherein the cooling results from emission of upconverted, anti-Stokes photoluminescence in the laser-induced cooling element.

11. An optomechanical resonator configured to indirectly cool via laser irradiance, the optomechanical resonator comprising:

a cantilever;

a cooling end of the cantilever, comprising a laser-induced cooling element;

wherein the laser induced cooling element comprises a microcrystal cooling material within the Mie-regime for light scattering; and

an attachment end of the cantilever, attached to a substrate;

wherein the laser-induced cooling element is attached to the cantilever by Van der Waals bonding.

12. The optomechanical resonator of claim 11, wherein the cantilever is formed from a material having a low absorption coefficient at a peak wavelength.

13. The optomechanical resonator of claim 11, wherein the cantilever is formed from a material selected from the group consisting of CdS, Si₃N₄, and SiO₂.

14. The optomechanical resonator of claim 11, wherein the laser-induced cooling element includes a material selected from the group consisting of Yb:YLiF₄, Yb:NaYF₄, Yb:LuLiF₄, Yb:KLuF₄, and Yb:KYF₄.

15. The optomechanical resonator of claim 11, wherein the optomechanical resonator is configured to be cooled greater than 20K when impinged with laser irradiance.

16. The optomechanical resonator of claim 11, wherein the laser irradiance has a peak wavelength in a range of about 1020 nm to about 1064 nm.

17. The optomechanical resonator of claim 11, wherein the cooling results from emission of upconverted, anti-Stokes photoluminescence in the laser-induced cooling element.

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